This invention relates to an operating control device for a winding type induction machine. In particular, it relates to an operating control device for a winding type induction machine wherein a winding type induction machine is operated with secondary excitation control of the secondary current performed using a PWM-controlled inverter.
For example in the most modern hydro-electric power stations, the need for a so-called variable-speed power generating system, wherein the induction machine is operated at the rotational speed at which the turbine efficiency is a maximum with respect to head changes or load changes has increased. In a hydro-electric generator plant with such a variable speed power generation system, a winding type induction machine is operated with variable speed. Such an operating control device for a winding type induction machine is disclosed in the literature: for example the system disclosed in paragraph 96, FIG. 3.2.11 of Collected Research Theses BMFT-FB-T84-154 (1) of the West German Bundesministerium fuer Forschung und Technologie. The system disclosed in this reference is known as a secondary excitation type variable speed power generating system, in which the primary side frequency is controlled to a constant irrespective of changes in rotational speed, by controlling the secondary current of the winding type induction machine using a frequency converter such as a cyclo converter or PWM controlled inverter. This system has the characteristic advantage that the capacity of the converter can be made small, so it can be applied in particular to large-capacity generating plants.
A generating plant is operated as part of a complex transmission system. In this transmission system, the transmission line has inductance, resistance and stray capacitance distributed along it. Shunt reactors and phase-advance capacitors are provided to improve the power factor. The impedance when the transmission system side is seen from the generating plant therefore has a frequency characteristic. Furthermore, when the transmission system is employed it is switched in a complex manner in response to power flow conditions, so this impedance characteristic is not fixed, but varies.
FIG. 1 is a view showing an example of how harmonic components, if such are present in the primary voltage of the induction machine, are transmitted to the output side of the generating plant, i.e. to the transmission system, when the generating plant is being switched into the transmission system.
FIG. 1 shows an example characteristic in which, due to the impedance characteristic of the transmission system, harmonic components present in the primary voltage of the induction machine are amplified with a peak as indicated by point a. This point is called the "antiresonance point" possessed by the transmission system. In a transmission system having such a characteristic, if the primary voltage of the induction machine were to contain even a slight harmonic component coinciding with point a, because of the antiresonance point, this component would be amplified, producing extreme distortion at the output voltage end of the generating plant. It is undesirable to operate the induction machine in such a condition of large voltage distortion, so this situation must be avoided.
The harmonic components contained in the primary voltage of the induction machine are practically proportional to the harmonic components contained in the secondary excitation voltage of the induction machine. If a PWM controlled inverter is used in the frequency converter for secondary excitation, the output voltage waveform of this inverter contains harmonic components, so these harmonic components have an effect on the harmonic components of the primary voltage of the induction machine.
FIG. 2 is a characteristic showing an example of a typical output voltage waveform of a three phase PWM controlled inverter for secondary excitation. In FIG. 2, v2u*, v2v*, and v2w* are voltage commands to the inverter, whose output frequency is f.sub.0. e.sub.s is a modulation triangular wave for PWM control, whose repetition frequency i.e. modulation frequency is f.sub.s. Switching elements constituting the inverter are controlled by comparing these voltage commands v2u*, v2v* and v2w* with modulation triangular wave e.sub.s, whereupon the fundamental frequency of the output voltage of the inverter is determined by the frequency f.sub.0 of the voltage commands and a typical PWM controlled inverter output voltage is obtained in which the repetition frequency of a pulse train that changes in square-wave fashion is determined by the frequency f.sub.s of the modulation triangular wave.
The harmonic components contained in this PWM controlled inverter output voltage vuv are expressed by the following equations, taking the frequency of the modulation triangular wave e.sub.s as f.sub.s and the frequency of the voltage commands v2u*, v2v* and v2w* as f.sub.0 : EQU f.sub.H =nf.sub.s .+-.kf.sub.0
where
n is an integer, 0 to .infin. PA1 and k is an integer, 0 to .infin. PA1 voltage distortion detection means that detects the voltage distortion of the primary voltage of the winding type induction machine; PA1 primary voltage phase detection means that detects the phase of the primary voltage of the winding type induction machine; PA1 rotor phase detection means that detects the rotational phase of a rotor of the winding type induction machine; PA1 secondary voltage phase calculation means that calculates the phase of the secondary voltage of the winding type induction machine based on the primary voltage phase detected by the voltage phase detection means and on the rotor phase detected by the rotor phase detection means; PA1 secondary current control means that calculates a voltage command signal for the PWM controlled inverter based on the secondary current of the winding type induction machine, the current command value, and the secondary voltage phase obtained by the secondary voltage phase calculation means; and PA1 gate control means that outputs to the PWM controlled inverter a gate control signal for performing PWM control by modulating the voltage command signal with a triangular wave of modulation frequency responsive to the magnitude of the voltage distortion signal, by inputting a voltage command signal from the secondary current control means and a voltage distortion signal from the voltage distortion detection means. PA1 a step wherein the voltage distortion of the primary voltage of the winding type induction machine is detected; PA1 a step wherein the rotational phase of a rotor of the winding type induction machine is detected; PA1 a step wherein the phase of the secondary voltage of the winding type induction machine is calculated based on the detected primary voltage phase and on the detected rotor phase; PA1 a step wherein a voltage command signal for the PWM controlled inverter is calculated based on the secondary current of the winding type induction machine, the current command value, and the calculated secondary voltage phase; and PA1 a step wherein there is output to the PWM controlled inverter a gate control signal for performing PWM control by modulating the voltage command signal with a triangular wave of modulation frequency responsive to the magnitude of the voltage distortion signal, by inputting the calculated voltage command signal and the detected voltage distortion signal. PA1 primary voltage phase detection means that detects the phase of the primary voltage of the winding type induction machine; PA1 rotor phase detection means that detects the rotational phase of a rotor of the winding type induction machine; PA1 secondary voltage phase calculation means that calculates the phase of the secondary voltage phase calculation means that calculates the phase of the secondary voltage of the winding type induction machine based on the primary voltage phase detected by the voltage phase detection means and on the rotor phase detected by the rotor phase detection means; PA1 secondary current control means that calculates a voltage command signal for the PWM controlled inverter based on the secondary current of the winding type induction machine, the current command value, and the secondary voltage phase obtained by the secondary voltage phase calculation means; and PA1 gate control means that outputs to the PWM controlled inverter a gate control signal for performing PWM control by inputting a voltage command signal form the secondary current control means and a transmission system changeover signal from a power command center and modulating the voltage command signal with a triangular wave of modulation frequency predetermined in accordance with the transmission system changeover signal. PA1 a step in which the phase of the primary voltage of the winding type induction machine is detected; PA1 a step in which the rotational phase of a rotor of the winding type induction machine is detected; PA1 a step in which the phase of the secondary voltage of the winding type induction machine is calculated based on the detected primary voltage phase and on the detected rotor phase; PA1 a step in which a voltage command signal for the PWM controlled inverter is calculated based on the secondary current of the winding type induction machine, the current command value, and the calculated secondary voltage phase; and PA1 a step of inputting the detected voltage command signal and a transmission system changeover signal from a power command center, modulating the voltage command signal with a triangular wave of modulation frequency that is predetermined in accordance with the transmission system changeover signal, and outputting a gate control signal for performing PWM control to the PWM controlled inverter.
As is clear for the above equation, the harmonic components contained in this PWM controlled inverter output voltage change depending on both the frequency f.sub.0 of the voltage commands and the frequency f.sub.s of the modulation triangular wave. In operation of an ordinary PWM controlled inverter, the frequency f.sub.s of the modulation triangular wave is fixed at a constant value, but the output frequency f.sub.0 changes over a wide range, so the harmonic components of the output voltage of the inverter change in a complex manner. Consequently, even in a variable speed generating system, the harmonic components contained in the output voltage change in a complex manner because the frequency of the PWM controlled inverter, which constitutes the secondary excitation power source, is controlled in a manner matching the rotational speed of the induction machine.
Accordingly, in the variable speed generating system employing a PWM controlled inverter, in the variable speed range, the modulation frequency f.sub.s of the secondary excitation voltage is determined such that no harmonic component corresponding to antiresonance point a of FIG. 1 is contained in the primary voltage of the induction machine. However, due to demands imposed in use of the transmission system, when operation is performed with the transmission system being switched over, the impedance characteristic of the transmission system changes, causing the antiresonance point to be displaced. This may result in harmonics contained in the primary voltage of the induction machine coinciding with the antiresonance point. In such cases, the distortion of the primary voltage of the induction machine is enormously increased. This may make it impossible to continue operation.
In this respect, in the system disclosed in the literature reference described above, voltage distortion of the primary voltage of the induction machine is not discussed, and, even when the transmission system is being switched over because of power system requirements, no measures are taken to ensure a stable induction machine primary voltage with little distortion. Development of an operating control device for a winding type induction machine wherein control is performed to make the primary voltage of the induction machine stable with little distortion even when the transmission system is being switched over because of power transmission requirements is now therefore being urgently called for.